LC sensors are well known in the art. For example, LC sensors may be used as electronic proximity sensors which are able to detect the presence of a conductive target. Some common applications of inductive sensors include, e.g., metal detectors and derived applications, such as rotation sensors.
FIG. 1 shows the basic structure of an LC sensor 10 in accordance with the prior art. In the example considered, the LC sensor 10 includes an inductor L and a capacitor C, which form a resonant circuit also called tank circuit. The arrangement further includes a power supply 102, such as a voltage source, and a switch 104. When the switch 102 is in a first position (as shown in FIG. 1), the capacitor C is charged up to the supply voltage. When the capacitor C is fully charged, the switch 102 changes position, placing the capacitor 102 in parallel with the inductor L so that it starts to discharge through the inductor L. This starts an oscillation between the LC resonant circuit 10.
From a practical point of view, the LC sensor 10 also includes resistive components R, which will dissipate energy over time. Accordingly, losses occur which will decay the oscillations, i.e., the oscillation is damped. Basically, such an LC sensor 10 may be used, e.g., to detect metallic objects, because the oscillation will be damped quicker in the presence of a metallic object (see, e.g., FIG. 2b) compared to an oscillation without a metallic object (see, e.g., FIG. 2a).
Generally speaking, the sensing component of an LC sensor 10 may be the inductor L, the capacitor C, and/or the resistor R. For example, the resistance R primarily influences the damping factor, while the L and C components primarily influence the oscillation frequency. Moreover, such an LC sensor 10 may also be created by simply connecting a capacitor C to an inductive sensor L, or an inductor L to a capacitive sensor C. However, the inductor L (with its dissipative losses) usually provides the sensing element.
FIG. 3a shows a possible embodiment for performing the LC sensing of the sensor 10 with a controller or control unit 20, such as a microcontroller, as described, e.g., in the documents Application Note AN0029, “Low Energy Sensor Interface—Inductive Sensing”, Rev. 1.05, 2013-05-09, Energy micro, or Application Report SLAA222A, “Rotation Detection with the MSP430 Scan Interface”, April 2011, Texas Instruments. In the example embodiment, the control unit 20 includes two pins or pads 202 and 204, and the LC sensor 10 is connected between these pins 202 and 204.
The control unit 20 includes a controllable voltage source 206 connected to the pin 202 to impose a fixed voltage VMID at this pin 202. For example, a digital-to-analog converter (DAC) is usually used for this purpose.
During a charge phase, the pin 204 is connected to ground GND. Accordingly, during this phase, the sensor 10 is connected between the voltage VMID and ground GND, and the capacitor C of the sensor 10 is charged to the voltage VMID. Next, the control unit 20 opens the second pin 204, i.e., the pin 204 is floating. Accordingly, due to the fact that the capacitor C of the sensor 10 has been charged during the previous phase, the LC resonant circuit 10 starts to oscillate as described above.
Thus, by analyzing the voltage (e.g., voltage V204) at pin 204, the oscillation may be characterized. In fact, as shown in FIG. 3b, the voltage at the pin 204 corresponds to a damped oscillation having a DC offset corresponding to the voltage VMID, imposed by the voltage source 206, i.e., the voltage VMID defines the middle point of the oscillation. Accordingly, the voltage VMID is usually set to half of the supply voltage of the control unit 20, e.g. VDD/2, to have the maximum range.
Often, the circuit also includes an additional capacitor C1 connected between the pin 202 and ground GND to stabilize the voltage signal VMID, and to provide the current boost required to charge the sensor. In order to analyze the signal at the pin 204 (see, e.g., FIG. 3a), the control unit 20 may include an analog-to-digital converter (ADC) 208 connected to the pin 204 to sample the voltage of the oscillation. Thus, based on the resolution and sampling frequency of the ADC 206, the whole oscillation may be characterized.
FIG. 4 shows an alternative approach in accordance with the prior art. Specifically, in the example considered, the control unit 20 comprises a comparator 210, which compares the voltage at the pin 204 with a reference signal, such as a reference voltage VRef. For example, this reference voltage VRef may be fixed, e.g. to VDD/2, or set via a digital-to-analog converter 212. For example, FIGS. 5a and 5b respectively show the oscillations with and without a metallic object in the vicinity of the sensor 10, along with a reference voltage VRef and the output CMP of the comparator 210. Generally speaking, the two approaches shown in FIGS. 3a and 4, i.e., the ADC 208 and comparator 210, may also be combined in the same control unit 20.
Thus, based on the foregoing, contactless motion measurement may be achieved by interfacing LC sensors directly with microcontroller integrated circuits (ICs). Such sensing may be useful, e.g., for metering systems (gas, water, distance, etc.). However, while handling and sampling sensors, microcontrollers (or MCUs) should reduce the power consumption to permit the development of battery-powered systems. Moreover, as MCU units are typically general-purpose, there is also the desire to reduce the silicon area consumption due to the specialized circuits used for the implementation of the above functionality. Thus, in LC sensor excitation and measurement techniques it maybe desirable to reduce consumption and cost, especially for battery powered applications as already mentioned.
Thus, a first problem is related to the use of dedicated low power analog components, e.g., for generating the voltage VMID and the internal reference voltage VRef, which results in a greater cost. A second problem is related to the digital-to-analog converter 210 that is to be both low power and fast enough to follow the damped oscillation. This leads to a significant power consumption per measurement, as well as challenging application constraints in battery-powered systems.
Furthermore, Process-Voltage-Temperature (PVT) variations are another important issue in battery powered systems where there are significant voltage changes. Indeed, the components which were described above could be affected by the PVT variations: sensors (damping factor, frequency, etc.); I/O pads current and resistance (excitation); and the comparator switching point, etc.